Connected: An Internet Encyclopedia
RFC 1583 - 12.1.2. Options

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### 12.1.2. Options

The Options field in the link state advertisement header indicates which optional capabilities are associated with the advertisement. OSPF's optional capabilities are described in Section 4.5. There are currently two optional capabilities defined; they are represented by the T-bit and E-bit found in the Options field. The rest of the Options field should be set to zero.

The E-bit represents OSPF's ExternalRoutingCapability. This bit should be set in all advertisements associated with the backbone, and all advertisements associated with non-stub areas (see Section 3.6). It should also be set in all AS external link advertisements. It should be reset in all router links, network links and summary link advertisements associated with a stub area. For all link state advertisements, the setting of the E-bit is for informational purposes only; it does not affect the routing table calculation.

The T-bit represents OSPF's TOS routing capability. This bit should be set in a router links advertisement if and only if the router is capable of calculating separate routes for each IP TOS (see Section 2.4). The T-bit should always be set in network links advertisements. It should be set in summary link and AS external link advertisements if and only if the advertisement describes paths for all TOS values, instead of just the TOS 0 path. Note that, with the T-bit set, there may still be only a single metric in the advertisement (the TOS 0 metric). This would mean that paths for non-zero TOS exist, but are equivalent to the TOS 0 path. A link state advertisement's T-bit is examined when calculating the routing table's non-zero TOS paths (see Section 16.9).

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Connected: An Internet Encyclopedia
RFC 1583 - 12.1.2. Options